You may be able to challenge this depending on how children services came to the figure in question.

Did they use a reasonable method is calculating this? How long is it for? I suggest you ask for a copy of the assessment to see how they reached this figure.

The Public Law Project’s guide to Support under Section 17 Children Act 1989 which addresses what help is available to families who are not entitled to mainstream welfare benefits or housing will be very helpful to you. On the last few pages of this guide there are details of organizations that might be able to provide in depth advice to you.